Blue organic electroluminescent devices

ABSTRACT

An organic EL device, comprising an anode and a cathode, and at least one organic luminescent medium containing a compound of benzazoles of the formula: ##STR1## wherein: n is an integer of from 3 to 8; 
     Z is O, NR or S; and 
     R and R&#39; are individually hydrogen; alkyl of from 1 to 24 carbon atoms, for example, propyl, t-butyl, heptyl, and the like; aryl or hetero-atom substituted aryl of from 5 to 20 carbon atoms for example, phenyl and naphthyl, furyl, thienyl, pyridyl, quinolinyl and other heterocyclic systems; or halo such as chloro, fluoro; or atoms necessary to complete a fused aromatic ring; 
     B is a linkage unit consisting of alkyl, aryl, substituted alkyl, or subsituted aryl which conjugately or unconjugately connects the multiple benzazoles together.

CROSS REFERENCE TO RELATED APPLICATION

Reference is made to commonly-assigned U.S. patent application Ser. No.08/699,903 filed concurrently herewith pending, entitled "ElectronTransporting Materials for Organic Electroluminescent Devices" by Shi etal, the teaching of which is incorporated herein.

FIELD OF THE INVENTION

This invention relates to organic electroluminescent (EL) devices. Morespecifically, this invention relates to the use of a novel class oforganic materials for producing efficient blue EL devices.

BACKGROUND OF THE INVENTION

Organic EL devices are known to be highly efficient and are capable ofproducing a wide range of colors. Useful applications such as flat-paneldisplays have been contemplated. Representative of earlier organic ELdevices are Gurnee et al U.S. Pat. No. 3,172,862, issued Mar. 9, 1965;Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, "DoubleInjection Electroluminescence in Anthracene," RCA Review, Vol. 30, pp.322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973.Typical organic emitting materials were formed of a conjugated organichost material and a conjugated organic activating agent having condensedbenzene rings. Naphthalene, anthracene, phenanthrene, pyrene,benzopyrene, chrysene, picene, carbazole, fluorene, biphenyl,terphenyls, quarterphenyls, triphenylene oxide, dihalobiphenyl,trans-stilbene, and 1,4-diphenylbutadiene were offered as examples oforganic host materials. Anthracene, tetracene, and pentacene were namedas examples of activating agents. The organic emitting material waspresent as a single layer medium having a thickness much above 1micrometer. Thus, this organic EL medium was highly resistive and the ELdevice required a relatively high voltage (>100 volts) to operate.

The most recent discoveries in the art of organic EL device constructionhave resulted in devices having the organic EL medium consisting ofextremely thin layers (<1.0 micrometer in combined thickness) separatingthe anode and cathode. Herein, the organic EL medium is defined as theorganic composition between the anode and cathode electrodes. In a basictwo-layer EL device structure, one organic layer is specifically chosento inject and transport holes and the other organic layer isspecifically chosen to inject and transport electrons. The interfacebetween the two layers provides an efficient site for the recombinationof the injected hole-electron pair and resultant electroluminescence.The extremely thin organic EL medium offers reduced resistance,permitting higher current densities for a given level of electrical biasvoltage. Since light emission is directly related to current densitythrough the organic EL medium, the thin layers coupled with increasedcharge injection and transport efficiencies have allowed acceptablefight emission levels (e.g. brightness levels capable of being visuallydetected in ambient light) to be achieved with low applied voltages inranges compatible with integrated circuit drivers, such as field effecttransistors.

Further improvement in organic EL devices such as color, stability,efficiency and fabrication methods have been disclosed in U.S. Pat. Nos:4,356,429; 4,539,507; 4,720,432; 4,885,211; 5,151,629; 5,150,006;5,141,671; 5,073,446; 5,061,569; 5,059,862; 5,059,861; 5,047,687;4,950,950; 4,769,292, 5,104,740; 5,227,252; 5,256,945; 5,069,975, and5,122,711; 5,366,811; 5,126,214; 5,142,343; 5,389,444; 5,458,977.

For the production of full-color EL display panel, it is necessary tohave efficient red, green and blue (RGB) EL materials. With theseprimary materials, an appropriate combination of their emissions willgenerate any desirable EL hues or colors, including white. Especiallyimportant is the production of blue EL materials, because, given anefficient blue EL material, it is possible to produce other EL colors bya downhill energy transfer process. For instance, a green EL emissioncan be obtained by doping into a host blue EL material with a smallamount of a green fluorescent sensitizing dye. This host-guest energytransfer scheme has been discussed in detail by Tang et al [U.S. Pat.No. 4,769,292]. Similarly, a red EL color can be produced by doping theblue EL host material with a red fluorescent dye. In a somewhatanalogous scheme, the fluorescent sensitizing dye may be placed outsidethe blue EL emitter to effect a shift in the EL emission wavelengths, asdiscussed by Imai [U.S. Pat. No. 5,015,999]. In this scheme, thesensitizing medium absorbs the blue photon generated by the blue ELemitter, which then emits at longer wavelengths.

In the present invention, it has been found that a class of novelorganic materials known as benzazoles is capable of producing highlyefficient blue electroluminescence. These materials can be used toproduce EL devices with a wide range of visible colors.

SUMMARY OF THE INVENTION

The present invention provides an efficient blue organic EL device,comprising an anode, a cathode, and an organic luminescent mediumbetween the anode and the cathode;

the luminescent medium contains a material or a mixture of materialsbelonging to the benzazoles of the formula I: ##STR2## wherein: n is aninteger of from 3 to 8;

Z is O, NR or S; and

R and R' are individually hydrogen; alkyl of from 1 to 24 carbon atoms,for example, propyl, t-butyl, heptyl, and the like; aryl or hetero-atomsubstituted aryl of from 5 to 20 carbon atoms, for example, phenyl andnaphthyl, furyl, thienyl, pyridyl, quinolinyl and other heterocyclicsystems;

or halo such as chloro, fluoro; or atoms necessary to complete a fusedaromatic ring;

B is a linkage unit consisting of alkyl, aryl, substituted alkyl, orsubstituted aryl which conjugately or unconjugately connects themultiple benzazoles together.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other advantages of this invention can be better appreciatedby reference to the following detailed description considered inconjunction with the drawings in which:

FIGS. 1, 2, and 3 are schematic diagrams of the multi-layer structuresof preferred EL devices of this invention.

FIG. 4 shows the spectral characteristic of an EL device described inExample 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An EL device 100 according to the invention is schematically illustratedin FIG. 1. The support is layer 102 which is an electrically insulatingand optically transparent material such as glass or plastic. Anode 104is separated from cathode 106 by an organic EL medium 108, which, asshown, consists of two superimposed layers of organic thin films. Layer110 located on the anode forms a hole-transport layer of the organic ELmedium. Located above the hole-transport layer is layer 112, which formsan electron-transport layer of the organic EL medium. The anode and thecathode are connected to an external AC or DC power source 114 byconductors 116 and 118, respectively. The power source can be pulsed,periodic, or continuous.

In operation, the EL device can be viewed as a diode which is forwardbiased when the anode is at a higher potential then the cathode. Underthese conditions, holes (positive charge carriers) are injected from theanode into the hole-transport layer, and electrons are injected into theelectron-transport layer. The injected holes and electrons each migratetoward the oppositely charged electrode, as shown by the arrows 120 and122, respectively. This results in hole-electron recombination and arelease of energy in part as light, thus producing electroluminescence.

The region where the hole and electron recombine is known as therecombination zone. The two-layer device structure is designedspecifically to confine the recombination at the vicinity near theinterface between the hole-transport and the electron-transport layerwhere the probability for producing electroluminescence is the highest.This recombination confinement scheme has been disclosed by Tang and VanSlyke [Applied Physics Letters, Volume 51, Page 913, 1987] and is doneby choosing carrier injecting electrodes of suitable work-functions andtransport materials of a proper carrier mobility. Away from thisinterface between the organic layers and in particular at or near theinjecting electrodes, the recombination of hole and electron wouldgenerally be much less radiative due to the effect of radiativequenching by a conducting surface.

Organic EL device 200 shown in FIG. 2 is illustrative of anotherpreferred embodiment of the invention. The insulating and transparentsupport is layer 202. The anode 204 is separated from the cathode 206 byan EL medium 208, which, as shown, consists of three superimposed layersof organic thin films. Layer 210 adjacent to anode 204 is thehole-transport layer. Layer 214 adjacent to cathode 206 is theelectron-transport layer. Layer 212 which is in between thehole-transport layer and the electron transport layer is the luminescentlayer. This luminescent layer also serves as the recombination layerwhere the hole and electron recombines.

The configurations of devices 100 and 200 are similar, except that anadditional luminescent layer is introduced in device 200 to functionprimarily as the site for hole-electron recombination and thuselectroluminescence. In this respect, the functions of the individualorganic layers are distinct and can therefore be optimizedindependently. Thus, the luminescent or recombination layer can bechosen to have a desirable EL color as well as a high luminanceefficiency. Likewise, the electron and hole transport layers can beoptimized primarily for the carrier transport property.

Organic device 300 is illustrative of yet another preferred embodimentof the present invention. The insulating and transparent support islayer 302. The anode 304 is separated from the cathode 306 by an ELmedium 308, which, as shown, consists of five superimposed layers oforganic thin films. Located on top of the anode layer 304 are, insequence, the hole-injection layer 310, the hole-transport layer 312,the luminescent layer 314, the electron-transport layer 316, and theelectron-injection layer 318. The structure of device 300 is similar todevice 200, except that a hole-injection layer and an electron injectionlayers are added to improve the injection efficiency of the respectiveanode and cathode. It is understood that an EL device may be constructedhaving either the hole or electron injection layer present in theorganic EL medium without unduly compromising the device performance.

The substrate for the EL devices 100, 200, and 300 is electricallyinsulating and light transparent. The light transparent property isdesirable for viewing the EL emission through the substrate. Forapplications where the EL emission is viewed through the top electrode,the transmissive characteristic of the support is immaterial, andtherefore any appropriate substrate such as opaque semiconductor andceramic wafers can be used. Of course, it is necessary to provide inthese device configurations a light transparent top electrode.

The composition of the organic EL medium is described as follows, withparticular reference to device structure 300.

A layer containing a porphyrinic compound forms the hole injecting layerof the organic EL device. A porphyrinic compound is any compound,natural or synthetic, which is derived from or includes a porphyrinstructure, including porphine itself. Any of the prophyrinic compoundsdisclosed by Adler, U.S. Pat. No. 3,935,031 or Tang U.S. Pat. No.4,356,429, the disclosures of which are here incorporated by reference,can be employed.

Preferred porphyrinic compounds are those of structural formula (II):##STR3## wherein Q is--N═ or--C(R)═;

M is a metal, metal oxide, or metal halide;

R is hydrogen, alkyl, aralkyl, aryl, or alkaryl; and

T¹ and T² represent hydrogen or together complete a unsaturated sixmember ring, which can include substituents, such as alkyl or halogen.Preferred six membered rings are those formed of carbon, sulfur, andnitrogen ring atoms. Preferred alkyl moieties contain from about 1 to 6carbon atoms while phenyl constitutes a preferred aryl moiety.

In an alternative preferred form the porphyrinic compounds differ fromthose of structural formula (I) by substitution of two hydrogens for themetal atom, as indicated by formula (III): ##STR4##

Highly preferred examples of useful porphyrinic compounds are metal freephthalocyanines and metal containing phthalocyanines. While theporphyrinic compounds in general and the phthalocyanines in particularcan contain any meal, the metal preferably has a positive valence of twoor higher. Exemplary preferred metals are cobalt, magnesium, zinc,palladium, nickel, and, particularly, copper, lead, and platinum.

Illustrative of useful porphyrinic compounds are the following:

Prophine

1,10,15,20-tetraphenyl-21H,23H-porphine copper (II)

1,10,15,20-tetrapheyl-21H,23H-porphine zinc (II)

Copper phthlocyanine

Chromium phthalocyanine fluoride

The hole transporting layer of the organic EL device contains at leastone hole transporting aromatic tertiary mine, where the latter isunderstood to be a compound containing at least one trivalent nitrogenatom that is bonded only to carbon atoms, at least one of which is amember of an aromatic ring. In one form the aromatic tertiary amine canbe an arylamine, such as a monarylamine, diarylamine, triarylamine, or apolymeric arylamine. Exemplary monomeric triarylamines are illustratedby Klupfel et al U.S. Pat. No. 3,180,730. Other suitable triarylaminessubstituted with vinyl or vinyl radicals and/or containing at least oneactive hydrogen containing group are disclosed by Brantley et al U.S.Pat. Nos. 3,567,450 and 3,658,520.

Another class of aromatic tertiary amines are those which include atleast two aromatic tertiary amine moieties. Such compounds include thoserepresented by structural formula (IV). ##STR5## wherein Q¹ and Q² areindependently aromatic tertiary amine moieties and

G is a linking group such as an arylene, cycloalkylene, or alkylenegroup of a carbon to carbon bond.

A preferred class of triarylamines satisfying structural formula (IV)and containing two triarylamine moieties are those satisfying structuralformula (V): ##STR6## where R¹ and R² each independently represents ahydrogen atom, an aryl group, or an alkyl group or R¹ and R² togetherrepresent the atoms completing a cycloalkyl group and

R³ and R⁴ each independently represents an aryl group which is in turnsubstituted with a diaryl substituted amino group, as indicated bystructural formula (VI): ##STR7## wherein R⁵ R⁶ are independentlyselected aryl groups.

Another preferred class of aromatic tertiary amines aretetraaryldiamines. Preferred tetraaryldiamines include two diarylaminogroups, such as indicated by formula (VII), linked through an arylenegroup: ##STR8## wherein Are is an arylene group,

n is an integer of from 1 to 4, and

Ar, R⁷, R⁸, and R⁹ are independently selected aryl groups.

The various alkyl, alkylene, aryl, and arylene moieties of the foregoingstructural formulae (IV), (V), (VII), can each in turn be substituted.Typical substituents including alkyl groups, alkoxy groups, aryl groups,aryloxy groups, and halogen such as fluoride, chloride, and bromide. Thevarious alkyl and alkylene moieties typically contain from about 1 to 6carbon atoms. The cycloalkyl moieties can contain from 3 to about 10carbon atoms, but typically contain five, six, or seven ring carbonatoms-e.g., cyclopentyl, cyclohexyl, and cycloheptyl ring structures.The aryl and arylene moieties are preferably phenyl and phenylenemoieties.

Illustrative of useful hole transport compounds are the following:##STR9##

The luminescent layer of the organic EL medium comprises of aluminescent or fluorescent material where electroluminescence isproduced as a result of electron-hole pair recombination in this region.In the simplest construction, the luminescent layer comprises of asingle component, that is a pure material with a high fluorescentefficiency. A well known material is tris (8-quinolinato) Aluminum,(Alq), which produces excellent green electroluminescence. A preferredembodiment of the luminescent layer comprises a multi-component materialconsisting of a host material doped with one or more components offluorescent dyes. Using this method, highly efficient EL devices can beconstructed. Simultaneously, the color of the EL devices can be tuned byusing fluorescent dyes of different emission wavelengths in a commonhost material. This dopant scheme has been described in considerabledetails for EL devices using Alq as the host material by Tang et [J.Applied Physics, Vol. 65, Pages 3610-3616, 1989; U.S. Pat. No.4,769,292].

An important relationship for choosing a fluorescent dye as a dopantcapable of modifying the hue of light emission when present in a hostmaterial is a comparison of their bandgap potential which is defined asthe energy difference between the highest occupied molecular orbital andthe lowest unoccupied molecular orbital of the molecule. For efficientenergy transfer from the host to the dopant molecule, a necessarycondition is that the bandgap of the dopant is smaller than that of thehost material. An advantage of using a blue host such as benzazole isthat its bandgap is sufficiently large to effect energy transfer to arange of commonly available fluorescent dyes emitting in the blue. Theseblue dopants includes coumarins, stilbenes, distyrylstilbenes,derivatives of anthracene, tetracene, perylene, and other conjugatedbenzenoids. Other dopants for EL emissions at longer wavelengths includecoumarins, rhodamines and other green or red emitting fluorescent dyes.

In the practice of the present invention, the host material forming theEL luminescent layer where light is emitted in response to electron-holerecombination comprises of a benzazole derivative or a mixture of thesederivatives represented by formula I. The dopants for the benzazoles mayinclude fluorescent dyes as described above. Efficient blueelectroluminescence can be readily obtained when this material is usedin layer 112 of FIG. 1, layer 212 of FIG. 2 or layer 314 of FIG. 3.##STR10## wherein: n is an integer of from 3 to 8;

Z is O, NR or S; and

R and R' are individually hydrogen; alkyl of from 1 to 24 carbon atoms,for example, propyl, t-butyl, heptyl, and the like; aryl or hetero-atomsubstituted aryl of from 5 to 20 carbon atoms, for example, phenyl andnaphthyl, furyl, thienyl; or halo such as chloro, fluoro; or atomsnecessary to complete a fused aromatic ring;

B is a linkage unit consisting of alkyl, aryl, substituted alkyl, orsubstituted aryl which conjugately or unconjugately connects themultiple benzazoles together. ##STR11## in which Z is O or C(CN)₂ ;##STR12## in which X is O, S, SO₂, CH₂, CHR, CRR', or NR; and R and R'are individually hydrogen; saturated aliphatic of from 1 to 24 carbonatoms, for example, propyl, t-butyl, heptyl, and the like; aryl of from6 to 20 carbon atoms, for example, phenyl and naphthyl; or halo such aschloro, fluoro, and the like; or R and R' individually comprise theatoms necessary to complete a fused aromatic ring; ##STR13## in which Zis O or C(CN)₂ ; and ##STR14## in which X and Y are individually O, S,SO₂, CH₂, CHR, CRR', or NR; and R and R' are individually hydrogen;saturated aliphatic of from 1 to 24 carbon atoms, for example, propyl,t-butyl, heptyl, and the like; aryl of from 6 to 20 carbon atoms, forexample, phenyl and naphthyl; or halo such as chloro, fluoro, and thelike; or R and R;individually comprise the atoms necessary to complete afused aromatic ring; ##STR15## in which Z is O or C(CN)₂ ; Z is O, S,SO₂, CHR, CRR', or NR; and R or R' are individually hydrogen; saturatedaliphatic of from 1 to 24

carbon atoms, for example, propyl, t-butyl, heptyl, and the like; arylof from 6 to 20 carbon atoms, for example, phenyl and naphthyl; or halosuch as chloro, fluoro, and the like; or R and R' individually comprisethe atoms necessary to complete a fused aromatic ring.

The following molecular structures constitute specific examples ofpreferred benzazoles satisfying the requirement of the invention:##STR16##

Preferred materials for use in forming the electron transporting layerof the organic EL devices of this invention are metal chelated oxinoidcompounds, including chelates of oxine itself (also commonly referred toas 8-quinolinol or 8-hydroxyquinoline). Such compounds exhibit both highlevels of performance and are readily fabricated in the form of thinfilms. Exemplary of contemplated oxinoid compounds are those satisfyingthe following structural formula: ##STR17## wherein Me represents ametal;

n is an integer of from 1 to 3; and

Z independently in each occurrence represents the atoms completing anucleus having at least two fused aromatic rings.

From the foregoing it is apparent that the metal can be monovalent,divalent, or trivalent metal. The metal can, for example, be an alkalimetal, such as lithium, sodium, or potassium; an alkaline earth metal,such as magnesium or calcium; or an earth metal, such as boron oraluminum. Generally any monovalent, divalent, or trivalent metal knownto be a useful chelating metal can be employed.

Z completes a heterocyclic nucleus containing at least two fusedaromatic rings, at least one of which is an azole or azine ring.Additional rings, including both aliphatic and aromatic rings, can befused with the two required rings, if required. To avoid addingmolecular bulk without improving on function the number of ring atoms ispreferably maintained at 18 or less.

Illustrative of useful chelated oxinoid compounds are the following:

Aluminum trisoxine [a.k.a, tris(8-quinolinol)aluminum]

Magnesium bisoxine [a.k.a. bis(8-quinolinol)-magnesium]

Indium trisoxine [a.k.a., tris(8-quinolinol)indium]

Lithium oxine (a.k.a., 8-quinolinol lithium)

The preferred materials for the multi-layers of the organic EL mediumare each capable of film-forming--that is, capable of being fabricatedas a continuous layer having a thickness of less than 5000 Å. Apreferred method for forming the organic EL medium is by vacuum vapordeposition. Extremely thin defect-free continuous layers can be formedby this method. Specifically, the individual layer thickness as low asabout 50 Å can be constructed while still realizing satisfactory ELdevice performance. It is generally preferred that the overall thicknessof the organic EL medium be at least about 1000 Å.

Other methods for forming thin films in EL devices of this inventioninclude spin-coating from a solution containing the EL material. Acombination of spin-coating method and vacuum vapor deposition method isalso useful for the fabrication of multi-layer EL devices.

The anode and cathode of the organic EL device can each take anyconvenient conventional form. Where it is intended to transmit lightfrom the organic EL device through the anode, this can be convenientlyachieved by coating a thin conductive layer onto a light transparentsubstrate--e.g., a transparent or substantially transparent glass plateor plastic film. In one form the organic EL devices of this inventioncan follow the historical practice of including a light transparentanode formed of tin oxide or indium tin oxide coated on a glass plate,as disclosed by Gurnee et al U.S. Pat. No. 3,172,862, Gurnee U.S. Pat.No. 3,173,050, Dresner "Double Injection Electroluminescence inAnthracene", RCA Review, Volume 30, pages 322-334, 1969; and DresnerU.S. Pat. No. 3,710,167 cited above.

The organic EL devices of this invention can employ a cathodeconstructed of any metal, including any high or low work function metal,heretofore taught to be useful for this purpose. Unexpected fabrication,performance, and stability advantages have been realized by forming thecathode of a combination of a low work function metal and at least oneother metal. For further disclosure, see U.S. Pat. No. 4,885,211 by Tangand Van Slyke, the disclosure of which is incorporated by referenceherein.

EXAMPLES

The invention and its advantages are further illustrated by the specificexamples as follows:

Example 1-Synthesis of 1,3,5-tis(N-phenyl-N-phenylamino)benzamide

To a solution of N-phenyl-1,2-phenylenediamine (16.6 g, 0.09 mol) in 100mL of N-methyl pyrrolidinone was added 1,3,5-benzenetricarbonyl chloride(8.0 g, 0.03 mol) in portion at room temperature under nitrogen. Thereaction mixture was stirred at room temperature for two hours thenraise the reaction temperature to 50° C. for another half hour. Aftercooling the reaction mixture was poured into 300 ml of cool water withstirring. The resulted precipitates were filtered and washed with water.After drying, the tribenzamide was collected to give 19.5 g. (yield92%).

Example 2-Synthesis of 2 2',2"-(1,3,5-phenylene)tris[1-phenyl-1H-benzimidazole](TPBI)

TPBI was prepared by heating 1,3,5-tris(N-phenyl-N-phenylamino)benzamidein 0.3 atm. nitrogen pressure at 280°˜295° C. for about one hour. Thepure TPBI (which can be used directly for cell fabrication) was obtainedby sublimation twice at 315° C. at 2 Torr pressure.

Example 3-EL device fabrication and performance

An EL device satisfying the requirements of the invention wasconstructed in the following manner. The organic EL medium has fourorganic layers, namely, a hole-injection layer, a hole transport layer,a luminescent layer, and an electron-transport layer

a) An indium-tin-oxide (ITO) coated glass substrate was sequentiallyultrasonicated in a commercial detergent, rinsed in deionized water,degreased in toluene vapor and exposed to ultraviolet light and ozonefor a few minutes.

b) A hole injection layer of copper phthalocyanine (150Å) was thendeposited on top of the ITO coated substrate by evaporation from atantalum boat.

c) Onto the copper phthalocyanine layer was deposited a hole transportlayer of N,N'-bis-(1-naphthyl)-N,N'-diphenylbenzidine (600Å), also byevaporation from a tantalum boat.

d) A luminescent layer of TPBI (200Å) was then deposited onto thehole-transport layer.

e) A electron-transport layer of Alq (375Å) was then deposited onto theluminescent layer.

f) On top of the Alq layer was deposited a cathode layer (2000Å) formedof a 10:1 atomic ratio of Mg and Ag.

The above sequence completed the deposition of the EL device. The devicewas then hermetically packaged in a dry glove box for protection againstambient environment.

The light output from this EL device was 200 cd/m² when it was driven bya current source of 20 mA/c m² and a bias voltage of 9.5 volts. The ELcolor is blue with 1931 CIE color coordinates of X=0.156 and Y=0.152.The EL spectrum shown in FIG. 4 has a peak emission at 460 nm. This ELspectrum indicates that EL emission originates from the luminescent TPBIlayer.

Example 4

This example illustrates an organic EL device where the EL mediumcontains a doped luminescent layer. The organic EL medium has fourorganic layers, namely, a hole-injection layer, a hole transport layer,a doped luminescent layer, and an electron-transport layer.

a) An indium-tin-oxide (ITO) coated glass substrate was sequentiallyultrasonicated in a commercial detergent, rinsed in deionized water,degreased in toluene vapor and exposed to ultraviolet light and ozonefor a few minutes.

b) A hole injection layer of copper phthalocyanine (150Å) was thendeposited on top of the ITO coated substrate by evaporation from atantalum boat.

c) Onto the copper phthalocyanine layer was deposited a hole transportlayer of N,N'-bis-(1-naphthyl)-N,N'-diphenylbenzidine (600Å), also byevaporation from a tantalum boat.

d) A layer of doped TPBI (200Å) was then deposited onto thehole-transport layer. The doped layer contains 0.8% perylene which wasco-deposited with the TPBI to form a uniform doped luminescent layer.

e) A electron-transport layer of Alq (300Å) was then deposited onto theluminescent layer.

f) On top of the Alq layer was deposited a cathode layer (2000Å) formedof a 10:1 atomic ratio of Mg and Ag.

The above sequence completed the deposition of the EL device. The devicewas then hermetically packaged in a dry glove box for protection againstambient environment.

The light output from this EL device was 310 cd/m² when it was driven bya current source of 20 mA/c m² and a bias voltage of 9.8 volts. The ELcolor is blue with 1931 CIE color coordinates of X=0.146 and Y=0.150.The EL spectrum has a peak emission at 456 nm. This EL spectrumindicates that EL emission originates from the perylene doped TPBI layerand is predominantly characteristic of the perylene dopant.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

Parts List

100 EL Device

102 Substrate

104 Anode

106 Cathode

108 Organic EL medium

110 Hole-transport layer

112 Electron-transport layer

114 External power source

116 Conductor

118 Conductor

120 Holes

122 Electrons

200 EL device

202 Substrate

204 Anode

206 Cathode

208 Organic EL medium

210 Hole-transport layer

212 Luminescent layer

214 Electron-transport layer

300 EL device

302 Substrate

304 Anode

306 Cathode

308 Organic EL medium

310 Hole-injection layer

Parts List cont'd

312 Hole-transport layer

314 Luminescent layer

316 Electron-transport layer

318 Electron-injection layer

What is claimed is:
 1. An organic EL device, comprising an anode and acathode, and at least one organic luminescent medium containing acompound of benzazoles of the formula: ##STR18## wherein: n is aninteger of from 3 to 8;Z is O, NR or S; and R and R' are individuallyhydrogen; alkyl of from 1 to 24 carbon atoms, for example, propyl,t-butyl, heptyl, and the like; aryl or hetero-atom substituted aryl offrom 5 to 20 carbon atoms for example, phenyl and naphthyl, furyl,thienyl, pyridyl, quinolinyl and other heterocyclic systems; or halosuch as chloro, fluoro; or atoms necessary to complete a fused aromaticring; B is a linkage unit consisting of alkyl, aryl, substituted alkyl,or subsituted aryl which conjugately or unconjugately connects themultiple benzazoles together.
 2. An organic EL device according to claim1, wherein the linkage unit B consists of: ##STR19##
 3. An organic ELdevice according to claim 1 wherein the linkage unit B consists of##STR20##
 4. An organic EL device according to claim 1 wherein thelinkage unit B consists of: ##STR21##
 5. An organic EL device accordingto claim 1 wherein the linkage unit B consists of: ##STR22##
 6. Anorganic EL device according to claim 1 wherein the linkage unit Bconsists of: ##STR23##
 7. An organic EL device according to claim 1wherein the linkage unit B consists of: ##STR24## in which Z is O orC(CN)₂.
 8. An organic EL device according to claim 1 wherein the linkageunit B consists of: ##STR25## in which X is O, S, SO₂, CH₂, CHR, CRR',or NR; and R and R' are individually hydrogen; saturated aliphatic offrom 1 to 24 carbon atoms, for example, propyl, t-butyl, heptyl, and thelike; aryl of from 6 to 20 carbon atoms, for example, phenyl andnaphthyl; or halo such as chloro, fluoro, and the like; or R and R'individually comprise the atoms necessary to complete a fused aromaticring.
 9. An organic EL device according to claim 1 wherein the linkageunit B consists of: ##STR26## in which Z is O or C(CN)₂ ; and ##STR27##in which X and Y are individually O, S, SO₂, CH₂, CHR, CRR', or NR; andR and R' are individually hydrogen; saturated aliphatic of from 1 to 24carbon atoms, for example, propyl, t-butyl, heptyl, and the like; arylof from 6 to 20 carbon atoms, for example, phenyl and naphthyl; or halosuch as chloro, fluoro, and the like; or R and R' individually comprisethe atoms necessary to complete a fused aromatic ring.
 10. An organic ELdevice according to claim 1 wherein the linkage unit B consists of:##STR28## in which Z is O or C(CN)₂ ; X is O, S, SO₂, CH₂, CHR, CRR', orNR; and R or R' are individually hydrogen; saturated aliphatic of from 1to 24 carbon atoms, for example, propyl, t-butyl, heptyl, and the like;aryl of from 6 to 20 carbon atoms, for example, phenyl and naphthyl; orhalo such as chloro, fluoro, and the like; or R and R' individuallycomprise the atoms necessary to complete a fused aromatic ring.
 11. Anorganic luminescent medium for use in an electronic device such as, forexample, an organic EL device, wherein the improvement comprises theorganic luminescent medium containing a compound of benzazoles of theformula: ##STR29## wherein: n is an integer of from 3 to 8;Z is O, NR orS; and R and R' are individually hydrogen; alkyl of from 1 to 24 carbonatoms, for example, propyl, t-butyl, heptyl, and the like; aryl orhetero-atom substituted aryl of from 5 to 20 carbon atoms for example,phenyl and naphthyl, furyl, thienyl, pyridyl, quinolinyl and otherheterocyclic systems; or halo such as chloro, fluoro; or atoms necessaryto complete a fused aromatic ring; wherein the linkage unit B is amoiety selected from one of the following groups; ##STR30## in which Zis O or C(CN)₂ ; or ##STR31## in which X is O, S, SO₂, CH₂, CHR, CRR',or NR; and R and R' are individually hydrogen; saturated aliphatic offrom 1 to 24 carbon atoms, for example, propyl, t-butyl, heptyl, and thelike; aryl of from 6 to 20 carbon atoms, for example, phenyl andnaphthyl; or halo such as chloro, fluoro, and the like; or R and R'individually comprise the atoms necessary to complete a fused aromaticring;or ##STR32## in which Z is O or C(CN)₂ ; and ##STR33## in which Xand Y are individually O, S, SO₂, CH₂, CHR, CRR', or NR; and R and R'are individually hydrogen; saturated aliphatic of from 1 to 24 carbonatoms, for example, propyl, t-butyl, heptyl, and the like; aryl of from6 to 20 carbon atoms, for example, phenyl and naphthyl; or halo such aschloro, fluoro, and the like; or R and R' individually comprise theatoms necessary to complete a fused aromatic ring; or ##STR34## in whichZ is O or C(CN)₂ ; X is O, S, SO₂, CH₂, CHR, CRR', or NR; and R or R'are individually hydrogen; saturated aliphatic of from 1 to 24 carbonatoms, for example, propyl, t-butyl, heptyl, and the like; aryl of from6 to 20 carbon atoms, for example, phenyl and naphthyl; or halo such aschloro, fluoro, and the like; or R and R' individually comprise theatoms necessary to complete a fused aromatic ring.